(1) Field of the Invention
The present invention relates to a vehicle state observer system which estimates a motion state of a vehicle.
(2) Description of the Related Art
A vehicle state observer system for use in an automotive vehicle is known. "Introduction to Modern Control Theory" by M. Shiraishi published on Nov. 25, 1987 from Keigaku Shuppan Co. in Japan teaches a vehicle state observer system which estimates a quantity of motion state of a vehicle.
The vehicle state observer system of the above type includes an observable state measuring unit which measures an observable quantity of state of a vehicle. A controlled variable detecting unit in the vehicle state observer system detects a controlled variable of the vehicle. An observer in the vehicle state observer system estimates an unobservable quantity of state of the vehicle from the measured observable quantity and from the detected controlled variable in accordance with a system matrix including a system variable.
The above vehicle state observer system estimates the quantities of state of the vehicle by maintaining a pole of the observer at a fixed point so as to optimize an error between the pole of the observer and a pole of a system.
Generally, a transfer function G(s) of the system is represented by the equation: G(s)=N(s)/D(s) where s is a complex number, N(s) is a numerator polynomial, and D(s) is a denominator polynomial. The numerator polynomial N(s) is indicative of the measured quantity, and the denominator polynomial D(s) is indicative of the controlled variable. A transfer function G'(s) of the observer is represented by the equation: G'(s)=N'(s)/D'(s) where s is a complex number, N'(s) is a numerator polynomial, and D'(s) is a denominator polynomial. The numerator polynomial N'(s) is indicative of the observed quantity, and the denominator polynomial D'(s) is indicative of the controlled variable.
The pole of the system mentioned above means a solution obtained by putting the denominator polynomial D(s) of the transfer function of the system equal to zero. The pole of the observer mentioned above means a solution obtained by putting the denominator polynomial D'(s) of the transfer function of the observer equal to zero.
If the pole of the observer is greater than the pole of the system, it is impossible to estimate the quantities of state of the vehicle. Therefore, in the conventional system, the pole of the observer is maintained at the fixed point which is smaller than the pole of the system.
However, the system variable which is included in the system matrix is changed, and the stability of the system is changed in accordance with the change of the system variable. The system variable is, for example, a vehicle speed, a vehicle weight, etc. Therefore, the pole of the system is changed in accordance with the change of the system variable. For example, when the system variable is changed, the system becomes more instable and the pole of the system becomes greater than before.
As described above, in the above vehicle state observer system, the pole of the observer is maintained at the fixed point. Thus, when the system variable is changed, the error between the pole of the observer and the pole of the system may be greater in accordance with the change of the system variable. If the error between the pole of the observer and the pole of the system is greater than a reference value, the accuracy of estimated quantities of state of the vehicle becomes low and the accuracy thereof may scatter.
Accordingly, the above vehicle state observer system may experience the scattering of the accuracy of estimated quantities of state of the vehicle when the system variable is changed.
In addition, a background technology of the vehicle state observer system of the present invention is disclosed in U.S. patent application Ser. No. 280,214, filed on Jul. 24, 1994 (corresponding to Japanese Laid-Open Patent Application No.7-89304), which has been assigned to the assignee of the present invention. The disclosure of the above application is incorporated by reference.